Meiosis and Sexual Reproduction. Homologous Chromosomes Chromosomes of each pair are similar in...
-
Upload
chloe-hampton -
Category
Documents
-
view
217 -
download
1
Transcript of Meiosis and Sexual Reproduction. Homologous Chromosomes Chromosomes of each pair are similar in...
Meiosis and
Sexual Reproduction
Homologous Chromosomes• Chromosomes of each pair are similar in
length and centromere position• Both carry genes controlling the same
inherited characteristics• We inherit one chromosome of each homologous pair from our mother and the other from our father…(Therefore, these are NOT identical!)
Meiosis• A type of cell division that…
– Occurs only in the sex organs• Sperm cells in men• Eggs in women
– Produces 4 genetically unique cells each with HALF the number of chromosomes as the parent cell
– Keeps the number of chromosomes in a species the same generation to generation
Karyotype
• Body cells are called somatic cells.• Germ cells develop into sex cells.
– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed to
offspring
Two Types of Cells
Haploid vs. Diploid
• Fertilization between egg and sperm occurs in sexual reproduction.
• Diploid (2n) cells have two copies of every chromosome.– Body cells are diploid.– Half the chromosomes come from each parent
Haploid (n) cells have one copy of every chromosome.
– Gametes are haploid.– Gametes have 22 autosomes and 1 sex
chromosome
Meiosis makes haploid cells from diploid cells
Some details of meiosis…• As with mitosis, meiosis begins after the
chromosomes have been duplicated during Interphase
• Meiosis consists of two distinct parts– Meiosis I – Meiosis II
Meiosis I
Homologous chromosomes exchange genetic material (cross over) and separate during anaphase
• Crossing over results in new combinations of genes
Meiosis IISister chromatids are separated much as
they are in mitosis except the resulting cells are haploid and not diploid
Key Differences between Mitosis and Meiosis
• Mitosis:– One cell division– No pairing of homologous
chromosomes– Produces two offspring cells
each with the same number of chromosomes as the parent cell (46)
– Produces genetically identical copies of the parent cell
• Meiosis:– Two cell divisions– Homologous
chromosomes pair– Produces four offspring
cells each with one set of chromosomes—half the number as the parent cell (23)
– Creates genetically unique cells different from the parent cell
• This is because homologous chromosomes exchange genetic information
Which process is this?
• Provides for growth, repair, and asexual reproduction
• Produces TWO daughter cells that are genetically identical to the parent cell
• Involves one division of the DNA• Diploid parent cells produce diploid daughter
cells• Happens in most cells of the body
Which process is this?
• Yields haploid daughter cells with only ONE member of the homologous chromosome pair
• Produces FOUR daughter cells that are genetically different from the parent cell
• Involves TWO divisions of the DNA• Exchange of genetic material occurs between
homologous chromosomes
Gametogenesis
• All sperm and eggs produced during gametogenesis are genetically unique.
• What would happen if humans created gametes that were all genetically identical to each other? (Think about your siblings)
• Twins—identical vs. fraternal (how does this occur?)
Trisomy 21—Down’s Syndrome
• Three copies of chromosome 21
Maternal Age
Incidence at Birth
20 1 in 1500
30 1 in 900
35 1 in 400
40 1 in100
45 1 in 30
Klinefelter’s Syndrome• Male—XXY • 1 in 1000 males• Sterile• Tall, thin, and slightly lower
IQ
Other Disorders
• Turner’s Syndrome—XO– 1 in 500 female births (most
are aborted before birth)– Very short, infertile, broad
chest and may have a webbed neck
• Edward’s Syndrome—Trisomy 18– 1 in 3000– Most babies die within the
first month to the first year